A wireless communication system enables two wireless communication devices to transmit data with each other in a wireless manner by means of different modulation technologies. However, the two sides of communication usually use asynchronous clock frequency systems, so a Carrier Frequency Offset (CFO) is generated between frequencies used at the two sides, leading to an error in signal decoding.
In a point-to-point (PtoP) system, receivers at both sides can solve the problem through respective frequency compensation technologies. In other words, both sides make compensation for a certain object. However, in a point-to-multipoint (PtoMP) system where one Base Station (BS) communicates with a plurality of User Equipments (UEs), the BS, as a host end, fails to adjust the receiving frequency for each UE. Therefore, conventionally, the UE proactively corrects the error of the transmitting frequency according to an error value of the receiving frequency, or the BS proactively notifies the UE to revise the transmitting frequency thereof.
However, in the case of a PtoMP system and that the UE moves with a high velocity, the conventional compensation method fails to compensate the error effectively. Referring to FIG. 1, which is a diagram showing an impact of a Doppler Effect on a frequency according to an embodiment of the disclosure. When the UE moves with a high velocity, a dramatic frequency difference is generated between a sending frequency and a receiving frequency of the same signal due to a Doppler Effect. Moreover, the frequency difference changes between positive and negative values when the two sides move closer or away from each other. Generally, the Doppler Effect may last 1 second to 10 seconds, and in this period, decoding in the wireless communication fails because the error changes dramatically in an extremely short time.
It is assumed that a transmitting frequency of the BS is fc, and a preset receiving frequency of the UE is fc′. The clock systems of the BS and the UE are asynchronous, so a frequency difference fd is generated (fd=fc−fc′). In a conventional mechanism that the UE makes the compensation proactively, the UE can set the transmitting frequency to fc1′+fd, trying to maintaining the receiving frequency of the BS at fc. However, if the UE moves towards the BS with a high velocity, the actual receiving frequency of the UE is fc+fdop, where fdop is an error caused by the Doppler Effect. Moreover, conventionally, the UE further increases the transmitting frequency to fc′+fd+fdop, so the receiving frequency of the BS is increased to fc′+fd+2*fdop due to the Doppler Effect, making the error greater.
In the conventional mechanism that the BS proactively notifies the UE to perform correction, the UE needs a long reaction time while the impact period of the Doppler Effect is relatively short, so this mechanism becomes an inefficient compensation manner. Therefore, in the case of a PtoMP system and that the UE moves with a high velocity, no compensation can be made for the Doppler Effect in the conventional technology.